Generally speaking, and without going into the details of the regulations applicable in a any particular part of the world, it is safe to say that such a beam must satisfy two fundamental conditions:
firstly it must have high light intensity along the direction its optical axis, which directionn corresponds with the longitudinal axis of the vehicle, thereby enabling visibility to be obtained at a considerable distance in front of the vehicle; and
secondly it must dissipate condsiderable flux in a relatively large solid angle about its axis so that the sides of the road are illuminated at ground level and up to a sufficient height for the vehicle driver to see all kinds of obstacles.
With references to the high intensity condition, it is known that the intensity of a light beam returned by a reflector is proportional to the frontal area of said reflector (Blondel's law).
Currently used main beam headlamps generally include a reflector whose reflecting surface is a paraboloid of revolution with an open front of relatively large diameter (not less than 10 cm) thus presenting a large frontal area (the area of the reflector projected onto a plane perpendicular to its optical emission axis), and being sufficiently enveloping in shape around the bulb disposed at its focus for it to recover as large a fraction as possible of the light flux emitted thereby. This construction provides a satisfactory compromise between range and performance.
However, main beam headlamps having a parabolic reflector of the above type suffer from the considerable drawback of a relatively large diameter. In practice this means that main headlamps must take up a relatively large amount of room in the vertical direction since even when the parabolic reflector is truncated between horizontal planes, some minimum height remains necessary in order to ensure that on-axis light intensity remains high enough (which is related to frontal area) and that general lighting also remains high enough (which is related to the total light flux reflected by the reflector).
FIGS. 1 and 2 are a diagrammatic front view and longitudinal section respectively through a prior art parabolic reflector, and show the results obtained thereby. Such a parabolic reflector R has a focus F at which a light source L is situated. The reflector is truncated top and bottom by two horizontal planes H.sub.1 and H.sub.2 which are disposed on either side of the optical axis OO passing through on the focus F. It can be seen that if the reflector R is given a height h.sub.1 (i.e. the distance between the plane H.sub.1 and H.sub.2), this height corresponds to a frontal area S.sub.1 and an emitted flux .phi..sub.1. If the height is then reduced to a value h, it can be seen from FIG. 1 that the frontal area is reduced to a value S thereby reducing performance. It can also be seen, form FIG. 2 that the light source disposed at the focus F of the reflector R is simultaneously less completely enveloped, thereby reducing the total flux reflected from .phi..sub.1 to .phi..
Unfortunately, the trend in modern motor car design is towards headlamps of reduced height (both for stylistic and for aerodynamic reasons), and this height may lie in the range 4 mm to 8 cm. For the reasons explained above, such reduced heights are generally incompatible with conventional headlamp design using parabolic reflectors.
One way of mitigating these drawbacks is to use elliptical reflectors having an opening of small size, with the light source being disposed at the inside focus of an elliptical reflector constituing a flux capturing system. Such a disposition has the major advantage of recovering an optimum amount of light flux from the source: it is obvious that an elliptical reflector occupies a much larger solid angle around the light source than can a parabolic reflector, so that for a given light source and for equal quantities of flux emission, the volume occupied by an elliptical reflector and the diameter of its open outher end are both much less than for a parabolic reflector. More precisely, inorder to implement an elliptical reflector acting as a flux capturing system, it is necessary to use a combination of an elliptical reflector co-operating with a light source located substantially at one of its focuses and sending light rays from said source in a converging beam towards its other focus, in conjunction with an optical deflector component which picks up the converging beam reflected by the elliptical reflector and which transforms it into a beam of substantially parallel rays for constituting the beam emitted by the headlamp, overall. Such an optical deflector component which co-operates with the elliptical reflector, may be a converging lens or a paraabolic reflector.
The present invention relates more particularly to the latter structure, i.e. to headlamps which make use of a combination of an elliptical reflector and of a parabolic reflector.
More precisely, the present invention concerns headlamps of the type comprising: a light source; a two-focus elliptical reflector having one of its focuses in the vicinity of the light source and close to the base of the reflector, and having its other focus in front of said source; and a parabolic reflector whose focus is close to the front of the elliptical reflector so that light rays emitted by the source and reflected by the elliptical reflector towards the focus of the parabolic reflector are finally reflected by the parabolic reflector as a beam of substantially parallel rays which constitute the headlamp beam.
Such a structure is known in the prior art, and is described, for example, in U.S. Pat. No. 1,981,328, in French patent No. 69.40151 (published under the number 2 067 925) and in published German patent application No. 3 317 149.
However, although such a structure is highly satisfactory in capturing light flux emitted by the light source, and in enabling elliptical reflectors having small diameter openings to be used, thereby enabling headlamps of small vertical extent to be provided, it suffers from the drawback of not giving very high light intensity on the vehicle axis. That is why it has only been suggested in the prior art for use in making dipped beam lamps, which do not require high intensity on the illuminated axis.
Preferred embodiments of the present invention provide a main beam headlamp of small vertical size using a structure of the above type but remedying the above drawbacks so as to provide both high intensity on the illumination axis and considerable light flux in a solid angle extending substantially all around the optical axis along which light is emitted.